The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all, or even any, of the problems brought forward in this section.
A cellular telecommunications network comprises, for example, a set of Public Land Mobile Networks (PLMN). An available PLMN is a PLMN for which a mobile device has found at least one cell and can read its PLMN identity.
A suitable cell of a PLMN is a cell on which the mobile device may camp for normal services. For example, for a Universal Terrestrial Radio Access (UTRA) cell and for a Global System for Mobile Communications (GSM) cell, the criteria are defined in the 3G partnership project (3GPP) standardization Technical Specification (TS) 25.304 (for 3G), in TS 43.022 (for 2G), and in TS 36.304 (for LTE).
For camping on a cell, the mobile device typically has to choose a cell and complete a cell selection/reselection process.
However, there are some cases where the mobile device is unable to camp on a suitable cell. For example, when the mobile device cannot find a suitable cell to camp on (e.g. in a PLMN approved by the operator is used), when no SIM (Subscriber Identity Module) or USIM (UMTS SIM) is inserted in the mobile device, or when the location registration has failed.
In such a case, the mobile device may attempt to camp on an acceptable cell irrespective of the PLMN identity. An acceptable cell is a cell that satisfies certain conditions, e.g. as specified in 3GPP TS 25.304. In particular, an acceptable cell may be a cell allowing the mobile device to make an emergency call. The mobile device may then enter a limited service state, in which it can only attempt to make emergency calls.
The limited service state and the transition between limited service state and normal service state procedures are described by the 3GPP TS 25.304.
In particular, the standard description implies that a mobile device in the limited service state monitors a paging channel, meaning reading the Paging Indicator Channel, to be aware if the mobile device is paged, as described in 3GPP TS 25.133. Paging channel monitoring of the serving cell may, for example, be performed periodically every 2.56 s (if no USIM) or from 0.08 s to 5.12 s (if an International Mobile Subscriber Identity (IMSI) is available).
The standard description also implies that in the limited service state the mobile device should be able to make an emergency call. To achieve this, the mobile device must be, at least, on an acceptable cell. In case of mobility, an acceptable cell can vanish. It is up to the mobile device to scan the radio environment and define the most suitable acceptable cell. S (cell selection criterion) and R (cell-ranking criterion) are tools described in the standard to characterize the suitability criteria of a cell in term of power and quality reception. A mobile device in the limited service state should also try to recover PLMN by triggering periodic PLMN scans.
The standard description further implies that a mobile device in the limited service state monitors system information of the cells it camps on, meaning reading the system information contained in the broadcast channel (BCH), as described in 3GPP TS 25.331.
These activities involve power consumption.
There is a need for improved methods and devices for optimizing power consumption of a mobile device when it is not registered to a cellular telecommunications network.